This invention relates to a temperature threshold sensing circuit comprising first and second bipolar transistors formed together as part of an integrated circuit and being biased so that the current density flowing in the first transistor is larger than that in the second transistor by a first known factor. The invention can be used in a circuit as set forth in the previous sentence which is suitable for integration alongside a power semiconductor device in order to guard against overheating of the power device. Such a combination may form a part of a so-called "smart power" integrated circuit (chip).
A temperature threshold sensing circuit as set forth in the opening paragraph is known from U.S. Pat. No. 4,733,162. The known circuit achieves a high accuracy by relying on the well-defined dependence of the voltage V.sub.BE across a semiconductor junction on the forward current density and the absolute temperature of the junction. The circuit detects the passing of a threshold temperature defined by the ratio of two integrated resistors.
A problem arises with the known circuit, and others known, for example, from U.S. Pat. No. 4,021,722 and U.S. Pat. No. 3,809,929, in that they are not suitable for integration in certain types of power chip, because of limitations of the process steps used in their manufacture. For example, processes used to manufacture the low voltage CMOS circuitry in "high-side" power switching chips, such as that described in European Patent Application EP-A2-0 294 882, may only provide npn bipolar transistors (unless extra process steps are added). Furthermore, these npn transistors may only be vertical devices used in a common-collector configuration because the n-type substrate which forms the drain of an n-channel power MOSFET (or it could be the collector of a vertical npn power bipolar transistor) is permanently connected to the positive supply rail.
Another problem is the large number of matched bipolar transistors that are involved in the temperature sensing in the known circuit, which all have to be at the same temperature. In smart power applications, where the source of heat is on the chip itself, very large temperature gradients can exist across the chip and positioning all of the temperature sensing transistors close enough to the source of heat for a rapid and accurate response to transients becomes difficult.